1. Field of the Invention
The present invention deals with the field of rotating thermal machines. It relates to a welded rotor for a thermal machine, and to a process for producing a rotor of this type.
A rotor of the abovementioned type is known, for example, from DE-A1 101 12 062.
2. Brief Description of the Related Art
Critical components, such as for example forged rotors or tubes or cast housings for high-temperature steam power plants or components of gas turbines or other turbomachines with operating temperatures of >700° C. have to be produced from nickel-base alloys having the required mechanical and creep strength at these temperatures.
First of all, at temperatures immediately above the use range of high temperature steels, nickel-base alloys which have a chemical composition enabling them to achieve the desired high-temperature properties in a simple way by solution-annealing at a temperature of typically close to 1000° C. followed by cooling are selected.
If the operating temperatures are higher still, even more complex nickel-base alloys are required such that they have the required mechanical and creep strength at even higher temperatures (typically >750° C.). These alloys have even more complex compositions, enabling them to achieve the desired properties through the formation of stable precipitations. Precipitations of this nature are produced by a heat treatment combined with precipitation hardening, which follows the preceding solution annealing and is generally carried out in a temperature range between 700 and 900° C.
Precipitation-hardenable nickel-base alloys of this type have the desired properties for applications in the temperature range>>700° C., but also have a number of drawbacks:
on account of the lack of suitable manufacturing devices and on account of their tendency to form cracks during production, they cannot be produced and machined in the sizes required for large rotors, tubes or housings;
on account of the broad solidification range of the alloys, they are difficult to weld without the formation of solidification cracks, which would make them unsuitable for use (cf. for example: High Temperature Materials for Power Engineering, Liege, 24–27 Sep. 1990, p. 1309, p. 1461, p. 1471 and p. 1481);
in particular, welding of the fully hardened material promotes the formation of cracks on account of the relative inability of the material to compensate for the differential expansions which occur during welding;
the alloys are expensive on account of the elements which are added to produce the high-temperature strength as a result of precipitation reactions.
In the case of large components, such as rotors, housings or the like, which are subject to high temperatures in operation, there are often regions in which the operating temperature is highest and regions in which the operating temperature is well below the highest operating temperature. For situations of this type, it has already been proposed some time ago to assemble (weld together) the components from a plurality of subsections consisting of a material which is adapted to the operating temperature of the corresponding section in accordance with the operating temperature distribution.
For example, it is known from DE-A1 199 53 079, in order to form a component, to weld together two parts made of high-alloy, heat-resistant martensitic/ferritic steels, austenitic steels or superalloys based on nickel, nickel-iron and cobalt, with at least one of the parts first of all being plated with a nickel-base filler in the joining region, the plated material then being subjected to a high grade heat treatment, and finally the parts being welded together using the same filler. In an exemplary embodiment which is explained in more detail, a first part made of IN706 (Inconel 706), for example a disk of a rotor (which is assembled from a plurality of disks) in the solution-annealed state, is plated with the filler SG-NiCr20Nb by means of submerged arc welding with wire. Then, the plated IN706 disk is subjected to a heat treatment required for the high-grade quality (stabilization anneal at 820+/−15° C., cooling to RT, precipitation hardening at 730+/−15° C., cooling to RT). The plated IN706 disk is then welded to a further plated disk formed from the high-alloy martensitic/ferritic steel St13TNiEL, the root layers being applied by means of TIG welding and the reinforcing layers being applied by means of submerged arc wire welding. Then, the welded component undergoes a stress-relief anneal at 610+/−15° C.
DE-A1 101 12 062 proposes a process for welding together two parts which are subject to different levels of thermal loading and are intended in particular for a turbomachine. The first part consists of steel and the second part consists of a nickel-base alloy. In this process, prior to the welding, first of all an interlayer, in which the additional elements which are present in the nickel-base alloy and are responsible for crack formation decrease progressively from the inside outward, is applied to the second part made of the nickel-base alloy. The second part preferably consists of IN625 (Inconel 625). The interlayer, which is built up from a plurality of individual layers, preferably consists of IN617 (Inconel 617).
Particularly in the case of rotors of large steam or gas turbines, the quality of the weld seams between the individual rotor disks is crucial for the mechanical properties of the rotor. It is therefore very important for the weld seams to be checked for defects using a nondestructive test method, such as ultrasound or X-ray testing, as accurately as possible and with a high resolution. If the rotor is constructed exclusively from individual rotor disks which are welded together, the encircling weld seams can only be tested without obstacle from the outside, since the spaces between adjacent rotor disks form a cavity which is closed off with respect to the outside. Although it has already been proposed (cf. for example U.S. Pat. No. 6,152,697) to provide inspection openings leading into the cavity in a welded rotor in order for the weld seams to be tested from the cavity side, these inspection openings allow only very restricted access which is not suitable for ultrasound and X-ray testing.
Testing of the weld seams from both sides is required in particular in the case of the ultrasound testing of weld seams which contain nickel-base alloys as fillers, since the ultrasound pulses are relatively strongly attenuated in these weld seams. In the case of X-ray testing, unimpeded access from both sides is required in order for the X-ray source to be arranged on one side and the associated receiving device, such as for example an X-ray film or the like, to be arranged on the other side.